Trying to solve in Maple:

restart;
f := 15;
fk := 7;
zm := 350;
ym := 200;
eps := 1 - fk^2*exp(-((z - zm)/ym)^2)/f^2;
dz := diff(z(x, bn), bn);
db := diff(b(x, bn), bn);
eq1 := diff(z(x, bn), x) = cot(b(x, bn));
eq2 := diff(b(x, bn), x) = subs(z = z(x, bn), -1/(2*eps)*diff(eps, z));
eq3 := diff(dz, x) = -bd/sin(b(x, bn))^2;
eq4 := diff(bd, x) = subs(z = z(x, bn), dz/(2*eps)*(diff(eps, z)^2/eps - diff(eps, z $ 2)));
sys := eq1, eq2, eq3, eq4;
cond := z(0, bn) = 0, b(0, bn) = bn, zd(0, bn) = 0, bd(0, bn) = 1;
dsolve({cond, sys}, [z(x, bn), b(x, bn)], numeric);
Error, (in dsolve/numeric/process_input) dependent variables must be functions of a single unknown, the independent variable. Got [z(x, bn), b(x, bn)]

What does "Error, (in dsolve/numeric/process_input) dependent variables must be functions of a single unknown, the independent variable. Got [z(x, bn), b(x, bn)] " mean?

How can this system be solved?